Cross-cluster search
editCross-cluster search
editThe cross-cluster search feature allows any node to act as a federated client across multiple clusters. In contrast to the tribe node feature, a cross-cluster search node won’t join the remote cluster, instead it connects to a remote cluster in a light fashion in order to execute federated search requests.
Cross-cluster search works by configuring a remote cluster in the cluster state and connecting only to a limited number of nodes in the remote cluster. Each remote cluster is referenced by a name and a list of seed nodes. When a remote cluster is registered, its cluster state is retrieved from one of the seed nodes so that up to 3 gateway nodes are selected to be connected to as part of upcoming cross-cluster search requests. Cross-cluster search requests consist of uni-directional connections from the coordinating node to the previously selected remote nodes only. It is possible to tag which nodes should be selected through node attributes (see Cross-cluster search settings).
Each node in a cluster that has remote clusters configured connects to one or more gateway nodes and uses them to federate search requests to the remote cluster.
Configuring cross-cluster search
editRemote clusters can be specified globally using cluster settings
(which can be updated dynamically), or local to individual nodes using the
elasticsearch.yml
file.
If a remote cluster is configured via elasticsearch.yml
only the nodes with
that configuration will be able to connect to the remote cluster. In other
words, federated search requests will have to be sent specifically to those
nodes. Remote clusters set via the cluster settings API
will be available on every node in the cluster.
This feature was added as Beta in Elasticsearch v5.3
with further improvements made in 5.4 and 5.5. It requires gateway eligible nodes to be on v5.5
onwards.
The elasticsearch.yml
config file for a cross-cluster search node just needs to list the
remote clusters that should be connected to, for instance:
|
The equivalent example using the cluster settings API to add remote clusters to all nodes in the cluster would look like the following:
PUT _cluster/settings { "persistent": { "search": { "remote": { "cluster_one": { "seeds": [ "127.0.0.1:9300" ] }, "cluster_two": { "seeds": [ "127.0.0.1:9301" ] }, "cluster_three": { "seeds": [ "127.0.0.1:9302" ] } } } } }
A remote cluster can be deleted from the cluster settings by setting its seeds to null
:
PUT _cluster/settings { "persistent": { "search": { "remote": { "cluster_three": { "seeds": null } } } } }
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Using cross-cluster search
editTo search the twitter
index on remote cluster cluster_one
the index name
must be prefixed with the cluster alias separated by a :
character:
GET /cluster_one:twitter/_search { "query": { "match": { "user": "kimchy" } } }
{ "took": 150, "timed_out": false, "_shards": { "total": 1, "successful": 1, "failed": 0, "skipped": 0 }, "_clusters": { "total": 1, "successful": 1, "skipped": 0 }, "hits": { "total": 1, "max_score": 1, "hits": [ { "_index": "cluster_one:twitter", "_type": "_doc", "_id": "0", "_score": 1, "_source": { "user": "kimchy", "date": "2009-11-15T14:12:12", "message": "trying out Elasticsearch", "likes": 0 } } ] } }
In contrast to the tribe
feature cross-cluster search can also search indices with the same name on different
clusters:
GET /cluster_one:twitter,twitter/_search { "query": { "match": { "user": "kimchy" } } }
Search results are disambiguated the same way as the indices are disambiguated in the request. Even if index names are identical these indices will be treated as different indices when results are merged. All results retrieved from a remote index will be prefixed with their remote cluster name:
{ "took": 150, "timed_out": false, "_shards": { "total": 2, "successful": 2, "failed": 0, "skipped": 0 }, "_clusters": { "total": 2, "successful": 2, "skipped": 0 }, "hits": { "total": 2, "max_score": 1, "hits": [ { "_index": "cluster_one:twitter", "_type": "_doc", "_id": "0", "_score": 1, "_source": { "user": "kimchy", "date": "2009-11-15T14:12:12", "message": "trying out Elasticsearch", "likes": 0 } }, { "_index": "twitter", "_type": "_doc", "_id": "0", "_score": 2, "_source": { "user": "kimchy", "date": "2009-11-15T14:12:12", "message": "trying out Elasticsearch", "likes": 0 } } ] } }
Skipping disconnected clusters
editBy default all remote clusters that are searched via cross-cluster search need to be available when
the search request is executed, otherwise the whole request fails and no search results are returned
despite some of the clusters are available. Remote clusters can be made optional through the
boolean skip_unavailable
setting, set to false
by default.
GET /cluster_one:twitter,cluster_two:twitter,twitter/_search { "query": { "match": { "user": "kimchy" } } }
{ "took": 150, "timed_out": false, "_shards": { "total": 2, "successful": 2, "failed": 0, "skipped": 0 }, "_clusters": { "total": 3, "successful": 2, "skipped": 1 }, "hits": { "total": 2, "max_score": 1, "hits": [ { "_index": "cluster_one:twitter", "_type": "_doc", "_id": "0", "_score": 1, "_source": { "user": "kimchy", "date": "2009-11-15T14:12:12", "message": "trying out Elasticsearch", "likes": 0 } }, { "_index": "twitter", "_type": "_doc", "_id": "0", "_score": 2, "_source": { "user": "kimchy", "date": "2009-11-15T14:12:12", "message": "trying out Elasticsearch", "likes": 0 } } ] } }
Cross-cluster search settings
edit-
search.remote.connections_per_cluster
-
The number of nodes to connect to per remote cluster. The default is
3
. -
search.remote.initial_connect_timeout
-
The time to wait for remote connections to be established when the node starts. The default is
30s
. -
search.remote.node.attr
-
A node attribute to filter out nodes that are eligible as a gateway node in
the remote cluster. For instance a node can have a node attribute
node.attr.gateway: true
such that only nodes with this attribute will be connected to ifsearch.remote.node.attr
is set togateway
. -
search.remote.connect
-
By default, any node in the cluster can act as a cross-cluster client and
connect to remote clusters. The
search.remote.connect
setting can be set tofalse
(defaults totrue
) to prevent certain nodes from connecting to remote clusters. Cross-cluster search requests must be sent to a node that is allowed to act as a cross-cluster client. -
search.remote.${cluster_alias}.skip_unavailable
-
Per cluster boolean setting that allows to skip specific clusters when no
nodes belonging to them are available and they are searched as part of a
cross-cluster search request. Default is
false
, meaning that all clusters are mandatory by default, but they can selectively be made optional by setting this setting totrue
.
Retrieving remote clusters info
editThe Remote Cluster Info API allows to retrieve information about the configured remote clusters, as well as the remote nodes that the cross-cluster search node is connected to.